- Chemical evidence in sediments of fossils that is found in modern sponges
- Accepted fossils from Ediacaran (pre-Cambrian)
- mollusks, cnidarians and sponges
- Animals diversify across Paleozoic, Mesozoic and Cenozoic
- punctuated by mass extinctions
Flattened body = most cells near environment or gastrovascular cavity
Breath through their skin
Invertebrates are not a monophyletic group!